The Blue Grotto (Italian: Grotta Azzurra) is a sea cave on the coast of the island of Capri, southern Italy. Sunlight, passing through an underwater cavity and shining through the seawater, creates a blue reflection that illuminates the cavern. The cave extends some 50 metres into the cliff at the surface, and is about 150 metres (490 ft) deep, with a sandy bottom.[1]

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The cave is 60 metres long and 25 metres wide. The cave mouth is two metres wide and roughly one metre high. For this reason, entrance into the grotto can only be achieved when tides are low and the sea is calm.[2] Without calm seas and low tides, the grotto becomes inaccessible, as the 1-metre entrance is impossible to pass. To enter the grotto, visitors must lie flat on the bottom of a small four-person rowboat. The oarsman then uses a metal chain attached to the cave walls to guide the boat inside the grotto. Swimming in the grotto is forbidden, both for safety reasons and to preserve water clarity.[3]

The Blue Grotto is one of several sea caves, worldwide, that is flooded with a brilliant blue or emerald light. The quality and nature of the colour in each cave is determined by the particular lighting conditions in that particular cave.

In the case of the Blue Grotto, the light comes from two sources. One is a small hole in the cave wall, precisely at the waterline, that is a meter and half in diameter. This hole is barely large enough to admit a tiny rowboat, and is used as the entranceway. In photographs taken from within the cave, the above-water half of this hole appears as a spot of brilliant white light. The second source of light is a second hole, with a surface area about ten times as large as the first, which lies directly below the entranceway, separated from it by a bar of rock between one and two meters thick. Much less light, per square meter, is able to enter through the lower opening, but its large size ensures that it is, in practice, the primary source of light.

As light passes through the water into the cave, red reflections are filtered out and only blue light enters the cave. Objects placed in the water of the grotto famously appear silver. This is caused by tiny bubbles, which cover the outside of the object when they are placed underwater. The bubbles cause the light to refract differently than it does from the surrounding water and gives off the silver effect.[3]

In part because of the dazzling effect of the light from the above-water opening, it is impossible for a visitor who is in one of the rowboats to identify the shape of the larger hole, the outline of the bar that separates the two holes, or the nature of the light-source, other than a general awareness that the light is coming up from underneath, and that the water in the cave is more light-filled than the air. A visitor who places a hand in the water can see it "glow" eerily in this light.

During Roman times, the grotto was used as the personal swimming hole of Emperor Tiberius as well as a marine temple. Tiberius moved from the Roman capital to the island of Capri in 27 AD. During Tiberius' reign, the grotto was decorated with several statues as well as resting areas around the edge of the cave. Three statues of the Roman sea gods Neptune and Triton were recovered from the floor of the grotto in 1964 and are now on display at a museum in Anacapri. Seven bases of statues were also recovered from the grotto floor in 2009. This suggests that there are at least four more statues lying on the cave's bottom.[4] The cave was described by the Roman historian Pliny the Elder as being populated with Triton "playing on a shell". The now missing arms on the recovered Triton statue – usually depicted with a conch shell, suggest that the statues recovered in 1964 are the same statues Pliny the Elder saw in the 1st century AD.[2] According to reconstructions of the original Blue Grotto, a swarm of Triton statues headed by a Neptune statue may have stood in the walls of the cave. The environmentalist association Marevivo aims to restore the Blue Grotto to its ancient glory by placing identical copies of the statues where they originally stood in the grotto. This project is being carried out in collaboration with the archaeological superintendence of Pompeii.[4]

At the back of the main cave of the Blue Grotto, three connecting passageways lead to the Sala dei Nomi, or "Room of Names", named for the graffiti signatures left by visitors over the centuries. Two more passages lead deeper into the cliffs on the side of island. It was thought that these passages were ancient stairways that led to Emperor Tiberius' palace. However, the passages are natural passages that narrow and then end further along.[2]

During the 18th century, the grotto was known to the locals under the name of Gradola, after the nearby landing place of Gradola. It was avoided by sailors and islanders because it was said to be inhabited by witches and monsters. The grotto was then "rediscovered" by the public in 1826, with the visit of German writer August Kopisch and his friend Ernst Fries, who were taken to the grotto by local fisherman Angelo Ferraro.

In 1842, Danish choreographer August Bournonville set the second act of his ballet Napoli in the Blue Grotto. In this fantastic tale, Golfo, the demon who rules the Blue Grotto, transforms the ballet's heroine, Teresina, into a Naiad.